The invention is related to resilient organic microballoon filled abradable materials, and in particular to abradable materials for use in the compressor sections of gas turbine engines, particularly in the low compressor section of such engines.
Modern large gas turbine engines have axial flow compressors, which include multiple circular airfoil arrays mounted at the periphery of rotatable disks. Adjacent each set of moving compressor airfoils is an array of stationary airfoils. The efficiency of such a compressor is strongly affected by air, which leaks around the ends of the moving airfoils. The typical approach to minimize such leakage is to provide an abradable air seal with which the outer ends of the compressor airfoil interact to minimize leakage. It is common in the art to find materials which comprise a silicone rubber matrix containing 15 to 50 weight percent of hollow glass microspheres combined with inorganic filler particles as the abradable air seal material.
The evolution of gas turbine engines has been in the direction of higher operating temperatures. Temperatures in the compressor section of the engine have increased moderately, while temperatures in the combustor in the turbine section have increased substantially over the past decades.
In gas turbine engines with glass microsphere containing seals, when the abradable seals abrade, the glass microspheres are carried through the combustor and turbine sections of the engine. In addition, the silicone matrix will often contain other inorganic fillers that can fuse with the rubbed debris and be detrimental to down-stream hardware. These inorganic fillers are commonly used for matrix reinforcement and thermo-oxidative stability. In modern engines, the operating temperatures in the combustor and turbine sections are sufficiently high to cause the glass microspheres to soften or melt. When the silicone rubber is abraded and is passed through the turbine, the silicone is oxidized to form water, carbon dioxide, and silica. The combination of the inorganic fillers, the glass microspheres, and silica may melt and/or fuse together. It has been occasionally observed that these melted or fused materials have adhered to engine components and/or have blocked air cooling holes. Blockage of cooling holes is detrimental to engine component longevity.
One way to correct this problem is described in U.S. Pat. No. 6,334,617, assigned to the assignee of the present invention. The abradable seal material in the U.S. Pat. No. 6,334,617 comprises a resilient matrix material containing hard organic filler particles. The elimination of glass microspheres and the reduction of inorganic filler particles greatly reduced the residue that led to blocked air cooling holes.
Regardless of the polymeric formulation used, large particle size inorganic materials, such as silica aggregates of about 300 nanometers, render the formulation non-abraidable.